Rechargeable batteries with high specific energy are desirable for solving imminent energy and environmental issues. Lithium-ion batteries have one of the highest specific energy among rechargeable batteries, but state-of-the-art technology based on intercalation mechanism has a theoretical specific energy of about 400 Wh/kg for both LiCoO2/graphite and LiFePO4/graphite systems. To achieve higher specific energy, new materials in both the cathode and anode are desired. Despite significant progress in the development of high capacity anode materials such as nanostructured silicon, the relatively low charge capacity of cathodes remains a limiting factor for commercializing rechargeable batteries with high specific energy. Current cathode materials, such as transition metal oxides and phosphates, typically have an inherent limit of about 300 mAh/g. On the other hand, sulfur-based cathodes have a theoretical capacity of about 1,673 mAh/g. Although its voltage is about 2.2 V vs Li/Li+, which is about 60% of conventional lithium-ion batteries, the theoretical specific energy of a lithium-sulfur cell is about 2,600 Wh/kg, which is about five times higher than a LiCoO2-graphite system. Sulfur also has many other advantages such as low cost and non-toxicity. However, the poor cycle life of lithium-sulfur batteries has been a significant hindrance towards its commercialization. The fast capacity fading during cycling may be due to a variety of factors, including the dissolution of intermediate lithium polysulfides (e.g., Li2Sx, 4≤x≤8) in the electrolyte, large volumetric expansion of sulfur (about 80%) during cycling, and the insulating nature of Li2S. In order to improve the cycle life of lithium-sulfur batteries, the dissolution of polysulfides is one of the problems to tackle. Polysulfides are soluble in the electrolyte and can diffuse to the lithium anode, resulting in undesired parasitic reactions. The shuttle effect also can lead to random precipitation of Li2S2 and Li2S on the positive electrode, which can change the electrode morphology and result in fast capacity fading.
Other approaches have been attempted to address material challenges of sulfur, such as surface coating, conductive matrix, improved electrolytes, and porous carbon. For example, graphene/polymer coating has been shown to yield a smaller capacity decay. Porous carbon is another approach to trap polysulfides and provide conductive paths for electrons. Nevertheless, in the case of porous carbon, for example, a large surface area of sulfur can still be exposed to the electrolyte, which exposure can cause undesired polysulfide dissolution. Moreover, lesser emphasis has been placed on dealing with the large volume expansion of sulfur during lithiation. This volume expansion of sulfur can cause a surrounding material, such as a coating, to crack and fracture, rendering the surrounding material ineffective in trapping polysulfides.
It is against this background that a need arose to develop the sulfur-based cathodes and related methods and electrochemical energy storage devices described herein.